Piping for toxic and hazardous gases

When processes change and manufacturing plants need to install any of several types of ultra-high-purity process gases, it’s the plant engineer who will be responsible for specifying an appropriate piping system. Learn about a sound system design that maximizes efficiency, purity and safety.

By Jeffrey S. Close

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When processes change and manufacturing plants need to install any of several types of ultra-high-purity process gases, it’s the plant engineer who will be responsible for specifying an appropriate piping system. A more critical aspect of ultra-high-purity gas piping systems is the way gases are delivered to the process tools.

No two specialty gas suppliers use the same language to identify gas purity or specifications. Common ground is found in reference to the term ultra-high-purity , or UHP, gases. Ultra-high-purity gases have a minimum purity of 99.999% or better. This is often referred to as “five nines” pure. This purity level is determined by subtracting the sum of the concentration of possible trace contaminants from 100%. Therefore, a minimum purity level of 99.999% is equivalent to a maximum contaminant level of 10 parts per million by volume.

Types of gases

Compressed gases typically found in a manufacturing facility are classified as oxidizers, inert or flammable. Oxidizers aren’t inherently flammable, but will contribute to combustion as an oxidant. Examples of oxidizers include air, chlorine, fluorine, nitric oxide and oxygen. Inert gases don’t take part in combustion processes nor do they react with other materials. An inert gas introduced into a room or confined space reduces the amount of oxygen and limits a combustion process such as a fire. Inert gases are commonly used in extinguishing systems when it’s important to avoid water damage. Examples of inert gases are argon, carbon dioxide, helium, nitrogen and xenon.

Flammable gases, together with air or oxygen in the right concentration, burn or explode if ignited. Examples of flammable gases include ammonia, ethylene, hydrogen, methane and silane.

Gas delivery

The three most common ways to supply ultra-high-purity gases to a facility are onsite gas production plants, bulk delivery from onsite storage tanks, and individual gas cylinders. Select the most reliable and cost-effective gas supply system by evaluating plant-specific factors such as onsite production capability, size and number of components in the design, number of connections to the equipment, and how much product needs to be stored for backup supply.

Onsite gas production plants typically are installed if you need large quantities of nitrogen, hydrogen or oxygen. The technologies these on-site plants use include membrane, cryogenic systems and pressure-swing adsorption systems. Each can provide an ultra-high-purity product at a much lower cost than other available delivery methods. Advantages to onsite gas production are the convenience of not having to place orders or wait for deliveries, and having no containers, cylinders or liquid gas to handle. Factors to consider when selecting an onsite gas production plant are the real estate required to install gas generating equipment and the additional piping required to distribute gas to the facility.

Bulk delivery to onsite storage tanks is the preferred source of gas for many facilities that require large quantities of liquid argon, carbon dioxide, hydrogen, nitrogen, oxygen and several semiconductor process gases. Tanker trucks transport these high-purity gases from the gas company’s production plants to bulk storage tanks located at the manufacturing facility site. A variety of storage tank sizes are available to suit individual customer’s needs. These storage tanks can be purchased outright or rented through a gas distribution company. An advantage to this gas delivery method is the ability to have large quantities of the gases used in high volume. Items to consider when selecting a bulk gas storage system include flow rate, pressure, usage pattern and site location.

Individual gas cylinders commonly are used when gas volume requirements are small. If the consumption rate increases, multiple cylinders can be manifolded together in banks to provide a greater source of supply and to reduce the amount of cylinder handling. These cylinders must be constructed to withstand high pressure and they must conform to specific U.S. Department of Transportation regulations. An advantage to this gas delivery method is the ability to have an unlimited quantity and variety of gases on site for different process requirements. Storage is a major factor to consider when multiple gas cylinders are onsite. Depending on the types and quantities of gases being stored, several separate gas rooms may be required.

Method of distribution

Producing and purifying a gas to advanced specifications is of no value unless it can be delivered to the point of use without introducing contaminants. Your system design must avoid sources of direct contamination and minimize sources of back-contamination and cross-contamination. System installation must follow rigorous specifications and procedures, including onsite inspection and additional cleaning.

The common piping material of choice within the industry is AISI 316L stainless steel tubing for the majority of gas delivery applications. Lines either can be single-wall or double-wall, depending on the hazard level of the gas involved. Gas delivery piping and manufacturing equipment is machined to a very smooth finish, electropolished and welded. The supplier carefully cleans components and tubing before assembling them into systems and equipment to remove contaminants from surfaces that are expected to come into contact with the process gas.

The supplier delivers piping connections and accessories in sealed bags to prevent contaminating them after cleaning. The fittings in systems and equipment are assembled either with metal-to-metal seals or by orbital welding. Metal-to-metal seals provide a leak-tight seal for services from vacuum to positive pressure. Orbital welding is performed with a power supply, orbital welding heads and fixture blocks. Together these components comprise a welding system that can join of pieces of stainless steel piping together precisely to eliminate mechanical seals or joints.

Once assembled, the systems and equipment must be leak-tight to keep out any contamination that may affect the process and to keep hazardous gases contained. The assembled systems are leak tested and dried down to low moisture levels before commissioning. In a piping system, it’s good practice to minimize the dead space associated with branch lines and individual components.

Depending on the process gas hazard level, a gas storage cabinet may be required. Highly toxic or flammable gases require a storage room provided with explosion relief and individually exhausted gas cabinets. The gas cabinet room should have nonrecirculated exhaust ventilation and should be under negative pressure in relation to the surrounding area. The gas cabinets should have self-closing doors and may require internal sprinklers. Sensors installed within the gas cabinets should connect to alarms that give warning in the event of a leak or exhaust system failure, as appropriate.

A valve manifold box or a valve manifold panel is used when a single gas cabinet feeds multiple tools or when mechanical joints are permitted. These manifold assemblies minimize total installed costs. They’re constructed in a panel box that encloses multiple ports of a single gas inlet line. The panel box is exhausted and sensors are installed within. These supply systems must take into account required safety and environmental precautions including isolation and shutdowns, as well as normal gas supply communications and alarms.

Certification of ultra-high-purity piping systems is required at each stage of construction, including the piping materials, installation and final testing. The certification process is demanding and it’s a critical step in ensuring the safety of the building before, during and after commissioning. Certification procedures should be established early on and well documented through to project completion. This process is the responsibility of the designer, contractor and plant engineer.

Safety

Personal safety is a top priority when it comes to process gases. Historically, most safety incidents involving process gases occur as individual cylinders are connected to and disconnected from the supply systems. Each facility will have its own recommended guidelines for cylinder changeout that include purging the existing lines, isolating valves, pressurizing tanks, and slowly bringing a cylinder back on line. Some cylinder changeouts require a full protective suit with headgear. Others can be handled simply with gloves. Understanding the safety precautions and procedures for each gas type is critical, and trained facility personnel should be the only ones to perform this service.

The safest approach is to take process gas as bulk delivery. This method typically is handled by the gas company and requires no facility staff involvement.

Facilities that need these process gases generally install a toxic gas monitoring system as an important safety precaution. This system monitors, alarms and shuts down manufacturing tools, gas dispensing equipment and facilities if there are any accidental releases of hazardous or toxic gases. Different sensor types to monitor multiple gases can be located at the air handling units’ return inlets and in the process exhaust ductwork, gas cabinets, valve manifold boxes and at tubing connections to equipment throughout the facility and its gas storage rooms.

Designing and installing an effective ultra-high-purity process gas system requires extensive research and planning to ensure the final system meets the plant’s needs. Suggested factors to consider are purity level, flow, pressure, tool connections, delivery method and expected future growth for each type of gas. Identifying these items during the earliest stages of design can help in the decision-making process. Because these systems are both unique and complex, many engineers find it helpful to outsource the design to a specialist in the process gas industry.

Jeffrey S. Close is TITLE at Pathfinder Engineers LLP in Rochester, N.Y. Contact him at jclose@PathfinderEngineers.com and 585-218-0730.

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